1. Field of the Invention
This invention relates to a method for rapidly breaking and hardening an asphalt emulsion by incorporating therein a water-soluble urethane prepolymer.
Surfaces of roads are abraded and worn by tire chains and studded snow tires, by heavy cars and trucks that travel thereon and by the effects of weather. Accordingly, with the increase of the amount of asphalt-paved roads, development of excellent repair and resurfacing binders for asphalt-paved roads has been demanded. It is preferred that such repair binders satisfy the following requirements. Namely, they should be liquid at ambient temperature about 10.degree. to 35.degree.C. and they should be appliable so as to form a layer having a thickness not exceeding 5 mm. After paving, they should show high resistance and durability against impacts, shocks and abrasions, and they should have a stickiness or adhesiveness, both to the existing asphalt road surface and to aggregate, so as to bond tightly the aggregate incorporated therein for rendering the road surface non-slippery. Further, they should harden rapidly so that the repaved road can be opened for motor traffic within about one hour after repaving.
2. Description of the Prior Art
As the conventional methods for hardening asphalt emulsions, there are known: a method in which, for example, an anionic surface active agent is incorporated into a cationic asphalt emulsion; a method in which a polyvalent metal ion such as calcium ion is added to an emulsion containing a soap as an emulsifier to thereby form a metal soap; and a method in which an acid stronger than a carboxylic acid is added to destroy or reduce the emulsifying activity of the soap used as an emulsifier. According to these conventional methods, the asphalt emulsion is broken, but the composition thereafter exists in a viscous mud-like state for a long time and it takes a long time for the composition to exhibit a sufficient bonding activity. Further, although the resurfaced road can be opened to motor traffic after a relatively short time, because the asphalt emulsion does not attain a sufficient stickiness to aggregate within a short time, scattering of the aggregate placed thereon occurs for a time period after opening of the paved road to motor traffic. Accordingly, if it is intended to form a skid-preventive surface on the paving, the aggregate for improving traction and preventing skids becomes scattered and the intended effect cannot be fully obtained. Therefore, according to the conventional hardening methods, an asphalt emulsion is used principally as binders for blacktopping compositions.